Leading mark

ABSTRACT

An instrument for marking leading lines, comprising a front screen and a behind screen (1,2,15,16), which include opaque lines (3) separated by transparent interstices (4), whereby an interference pattern, a so-called moire pattern, arises when viewing the instrument. The screens have different division, i.e. different distances between the opaque lines. 
     According to the invention, each of the screens (1,2,15,16) comprises a plate, which at its center is bent to an obtuse angle (v), which screen plates are positioned by means of attachment means (6,7,8) so that the obtuse angles (v) face toward each other, in which case the front screen plate (1) is provided with a denser screen division than the behind screen plate (2), or alternatively so that the obtuse angles (v) face away from each other, in which case the front screen plate (15) is provided with a sparser screen division than the behind screen plate (16).

This invention relates to a leading mark, more precisely to aninstrument for marking a definite direction for an observer.

Instruments utilizing light interference technique for indicating aplane are previously known a.o. through the Swedish patentspecifications Nos. 7611513-8 and 354 354, which relate to an instrumentutilizing moire technique for bringing about for an observer theindication of a certain horizontal or vertical plane.

The instrument according to said patents comprises three screens, eachof which consists of opaque lines separated by transparent interstices.When viewing the instrument, indication is obtained that the observer isoutside a certain plane defined by the instrument, in that aninterference pattern, a so-called moire pattern arises which includeslines with a discontinuous angular deviation.

In the Swedish patent specification No. 354 354 an instrument isdisclosed, at which two screen plates are provided angularly relativeone another, and the division of the screens, i.e. the number of opaquelines per length unit perpendicular to the opaque lines, are selected sothat an interference pattern arises which is to be read in respect ofthe symmetry of the pattern.

An accurate determination of the indicated plane by means of such apattern is extremely difficult.

The instruments above referred to per se render a very high accuracy,but this type of indication does not meet certain requirements forleading marking.

One of these requirements is that upon viewing the instrument it must beclearly apparent therefrom, in a simple way, preferably by marking inthe form of arrows, in which direction a deviation from the leading lineprevails.

A further requirement is very uniform quality in respect of the activeelements of the instruments and of their location, in order thereby toobtain identical interference patterns produced by different instrumentsfor the same deviation from the leading line.

The present invention solves the aforesaid problems by a combination ofthe design of the screens and their attachment means. The screensaccording to the invention are formed so that the precision of theposition in a first direction is less essential, and so that thenecessary precision in respect of their position in a directionperpendicular to the first direction is obtained automatically by theattachment means.

The present invention relates to an instrument for marking leadinglines, comprising a front screen and a behind screen, which includeopaque lines separated by transparent interstices, in such a manner,that upon viewing the instrument an interference pattern, a so-calledmoire pattern, arises, said screens having different division, i.e.different distances between the opaque lines. The invention ischaracterized in that each of said screens comprises a plate, whichcentrally is bent to form an obtuse angle, that two such screen platesare positioned by attachment means so that the obtuse angles either facetoward each other, in which case the front screen plate is provided witha screen division of greater density than the behind screen plate, oralternatively so that the obtuse angles face away from each other, inwhich case the front screen plate is provided with a screen divisionmore sparse than the behind screen plate.

The invention is described in greater detail in the following, withreference to the accompanying drawings, in which

FIGS. 1 and 6 schematically show screen settings according to theinvention,

FIG. 2 shows the instrument seen along its stated leading line,

FIG. 3 is a view where the right-hand side piece in FIG. 2 has beenremoved,

FIG. 4 shows the instrument observed to the left of its stated leadingline,

FIG. 5 shows the instrument observed to the right of its stated leadingline.

In FIG. 1 a screen setting according to the invention is shownschematically, where each screen 1,2 includes opaque lines 3 separatedby transparent interstices 4. The screens 1,2 are arranged overlappingso that an observer viewing the screens from a direction indicatedschematically by the arrow 5 looks through front and behind screens.Hereby an interference pattern, a so-called moire pattern, is formedwhen viewing the instrument.

The screens 1,2 have different division, i.e. different numbers ofopaque lines per length unit perpendicular to the opaque lines.

Each screen 1,2 comprises a plate, which at its centre is bent to forman obtuse angle v, see FIG. 3.

The angle v may be between 120° and 170°, or below or above thesevalues, depending on the desired sensitivity, as will be explainedbelow.

According to one embodiment of the invention, two such screen plates 1,2are positioned so that the obtuse angles face toward each other, asshown in FIG. 1, and according to a second embodiment said plates arepositioned so that the said angles face away from each other, as shownin FIG. 6.

According to the firstmentioned embodiment, the front screen plate 1 isprovided with a division, which is more dense than that of the behindscreen plate 2.

For employing the instrument within a preferred distance interval fromabout 25 to 250 m, the opaque lines 3 of the front screen, for example,may have a width of 3 mm while the distance between two adjacent opaquelines, i.e. the transparent interstice 4 has a width of 1.5 mm. Theopaque lines 3 of the behind screen 2 also have a width of 3 mm whileits transparent interstices have a width of 1.75 mm. These measurementvalues are stated only as example.

The screen plates 1,2 are supported by attachment means, which comprisetwo lateral pieces 6,7, for example metal sheets or corresponding ones,which are arranged in parallel with each other and perpendicularly tothe plane surfaces of the screen plates 1,2.

The screen plates 1,2 are clamped between the lateral pieces 6,7 bymeans of set bolts 8 or corresponding means extending between thelateral pieces 6,7.

The lateral pieces 6,7 are provided with indications in the form ofupright pins 9 or the like arranged in pairs, or in the form of grooves.Between each pair of indications the edge 10 of a screen plate 1,2extends.

The screen plates 1,2 are rectangular or square plane plates, whichafter having been provided with a screen extending in parallel with twosides are bent to the angle v perpendicular to the screen lines. Thisimplies that each screen plate 1,2 has parallel sides where every screenline is in parallel with two opposed sides.

Each of the screen plates 1,2 includes in respect of the plate asymmetrically located opaque line 3 or a transparent interstice 4.

When, thus, the screen plates 1,2 are positioned as shown schematicallyin FIG. 1 and are surrounded by the lateral pieces 6,7 and insertedbetween the pairs of indications 9, the screens are located so thattheir screen lines 3 are in parallel with each other, and so that saidsymmetrically located opaque line 3 or said transparent interstice 4 arelocated directly in front of each other, seen in a direction ofobservation which is perpendicular to the plane surfaces of the screenplates 1,2.

It is extremely essential that the screen lines 3,4 are in parallelrelative to each other, and that the position of said symmetric line 3or interstice 4 is such as stated above, in order to give rise to theformation of an interference pattern consisting of dark strips 11 inparallel relative to each other, as shown in FIG. 2, when the instrumentis viewed perpendicularly to the plane surfaces of the screen plates 1,2in a direction of observation indicated by the arrow 12 in FIG. 3, i.e.along the leading line shown by the instrument.

The interference pattern or the accuracy expressed as the deviation fromthe leading line where the interference pattern shown in FIG. 2 isdisturbed, is affected relatively little by the fact that the screenplates 1,2 are slightly offset in vertical direction from the positionshown in FIG. 3.

The first direction mentioned in the introductory portion above is to beunderstood as a vertical displacement in FIG. 2 of one screen platerelative to the second one. The second direction mentioned above in theintroductory portion is to be understood as a displacementperpendicularly to the plane of the paper in FIG. 2, so that theposition of the symmetric lines is not the one intended.

Due to the fact that the screen plates 1,2 are plane rectangular plates,which are provided with screen lines 3,4 extending in parallel with twosides and thereafter are bent to a definite angle, thus, a high accuracyof the instrument, also in respect of the angle v, is obtained when thescreens are clamped between the plane lateral pieces 6,7.

The afore-described screen plates 1,2, in a setting according to FIGS. 1and 3, yield an interference picture as shown schematically in FIG. 2when the instrument is viewed along its leading line, i.e. in adirection perpendicular to the plane surfaces of the screen plates 1,2.

When the instrument according to FIGS. 1 and 3 is viewed from adirection located to the left of the leading line, the interferencepattern shown in FIG. 4 is formed. It also consists of dark strips 13,but these strips 13 form arrows, which indicate the correction directionfor coming to the leading line. When the instrument is viewed from adirection located to the right of the leading line, in a correspondingmanner the interference pattern shown in FIG. 5 is formed which consistsof arrows formed of dark strips 14 which also indicate the correctiondirection.

By designing the screen plates 1,2 and by positioning them as statedabove, a simple and accurate instrument for marking leading lines andfor indicating the correction direction is obtained.

When the screen plates are set as shown in FIG. 6, where the screenplates are designated 15,16, the front screen plate 15 is provided witha screen having a division sparser than that of the behind screen 16.The screen divisions here may be such as stated above. Hereby theinterference patterns in FIGS. 4 and 5 form in connection to thedeviations from the leading line described in these Figures.

The angle v affects the sensitivity of the instrument. At a small anglev the interference pattern consisting of arrows forms for a very smalldeviation from the leading line. When the angle v is greater, thissensitivity is lower. Practical tests have shown that for an intendeddistance interval of 25 to 250 m an angle v of 145° is to be preferred.

In the foregoing the terms leading line and leading mark have been used.These terms were used in order to express that the instrument alone,i.e. without using a further instrument or mark, shows a specific linewhen viewing the instrument, and also to express that the instrument isintended to be preferably used for indicating leading lines, especiallyfor navigation.

According to FIGS. 2 and 3, the instrument comprises, in addition towhat was mentioned above, a front piece 17 of a transparent material,for example a plastic material, and a rear piece 18, preferably of sheetmetal. It further comprises preferably an inner wall 19 of transparentmaterial, for example plastic material, which inner wall is intended toserve as a diffusor for the light emitted by one or several lightsources, preferably fluorescent tubes 20, which are located between theinner wall 19 and the rear piece 18. In order to increase the contrastof the interference pattern, the light sources are capable to emityellow light. Alternatively the light sources 20 are capable to emitwhite light while the inner wall 19 is coloured so as to emit yellowlight to the screen plates 1,2.

An electric heating element 21 of known type further may be provided forheating the instrument and thereby to prevent frost formation when theinstrument is placed outdoors. When using a heating element 21,preferably a thermostat 22 is located in the upper part of theinstrument for controlling the temperature within the instrument.

The present invention can be used for a great number of purposes, forexample leading marking for navigation. A special application is toarrange two instruments spaced from one another for marking each itsleading line, which lines intersect each other in a point. Such anarrangement can be used, for example, for docking-in or parkingaircraft, in which case the pilot follows the leading line of one of theinstruments to the point where he also is on the leading line of thesecond instrument, whereby the intended position for the aircraft isobtained.

The present invention, thus, renders possible a simple instrument, whichhas a uniform quality in respect of the interference patterns producedby various instruments, and which shows a leading line and, in addition,the correction direction in the case when the observer is not on theleading line. The attachment means, of course, can be varied, and thedesign of the screens can be altered without abandoning the idea of theinvention.

The present invention, thus, must not be regarded restricted to theembodiments set forth above, but can be varied within the scope of theattached claims.

I claim:
 1. An instrument for marking a leading plane by generating amoire pattern viewed by an observer of the instrument, said instrumentcomprising two screens arranged in opposing relationship for observationto form a front screen and a rear screen, each of said screens includingparallel opaque lines separated by transparent interstices, said screenshaving differing distance between the respective opaque lines, whereineach of said screens is defined by a plate bent along a line of bendperpendicular to said opaque lines to define two screen portions formingan obtuse angle therebetween, said plates being joined by attachmentmeans so that said respective lines of bend are parallel and therespective obtuse angles are positioned in opposing alignment, forproducing a moire pattern forming arrowheads which point in thedirection of motion required by the observer for approaching the leadingplane.
 2. The instrument of claim 1, wherein said obtuse angles arealigned facing one toward the other, and said front screen has a largerdistance between said respective opaque lines than the respectivedistance between the opaque lines in said rear screen.
 3. The instrumentof claim 1, wherein said respective obtuse angles face away one from theother, and said front screen has a smaller distance between saidrespective opaque lines than the respective distance between the opaquelines in said rear screen.
 4. The instrument of claim 1, wherein saidattachment means comprise a pair of spaced sheets positioned parallelone with the other and perpendicular to the planes of the said screenportions, opposing side edges of said screen portions being clampedbetween said sheets, said sheets having securing means to secure saidrespective side edges therebetween.
 5. The instrument of claim 4 whereinsaid securing means comprise pins projecting perpendicularly from saidrespective sheet.
 6. The instrument of claim 1, wherein said obtuseangles are within the range of 120 to 170 degrees.
 7. The instrument ofclaim 1, wherein each of said obtuse angles is approximately 145degrees.
 8. The instrument of claim 4, wherein said obtuse angles arewithin the range of 120 to 170 degrees.
 9. The instrument of claim 4,wherein each of said obtuse angles is approximately 145 degrees.
 10. Theinstrument of claim 1, wherein said line of bend is centrally located ineach of said respective bent plates.